Integrated circuits are manufactured by processes such as sputtering, ion implantation, and chemical vapor deposition that create successive layers of thin films of electrically conducting, electrically non conducting, and electrically semiconducting materials on a substrate. Techniques such as those involving photo resistive chemicals, masks, ultraviolet light, and etching are used to create multiple layers of these materials in patterns that create the electronic pathways that form an integrated circuit. Typically, hundreds of identical integrated circuits are created simultaneously in a geometrically repetitive pattern on a single substrate. Often these processes are conducted at high temperatures and other process conditions that induce mechanical stress in the substrates. These stresses can eventually cause cracking, delaminating, voiding, and other defects in the integrated circuits. Such defects may not become apparent until much later in the manufacture of the integrated circuit, so it is important to measure the stress levels at successive stages in the fabrication process and cull defective substrates before investing more resources to complete their fabrication.
Excessive stresses in the thin films distort the flatness of the substrate. Various techniques have been developed over the years to measure this distortion and use that information to compute the residual stress. Most of these techniques employ a laser beam that impinges the surface of the substrate and deposited materials. The expected location of the reflected beam is calculated based upon the geometry of the apparatus. The displacement of the actual reflected beam from its expected location is used to map the topography of the substrate and deposited materials. Differences detected between successive maps of topographical information are typically input into the well known “Stoney's equation” to calculate the induced stress. A significant problem with this process is that patterns in the deposited materials distort the reflection angle of the laser beam, making it difficult to interpret the reflection displacement data.
What is needed is a system to overcome the problems with interpreting the reflection angles of a laser beam in substrate stress analysis equipment where distortions in the reflection angles are caused by deposition patterns on the substrates.